Chemical activation of olive tree pruning to remove lead(II) in batch system: Factorial design for process optimization

b i o m a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 2 2 e3 3 2

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Chemical activation of olive tree pruning to remove lead(II) in batch system: Factorial design for process optimization M. Calero 1, A. Ronda*, M.A. Martı´n-Lara 2, A. Pe´rez 2, G. Bla´zquez 3 Department of Chemical Engineering, University of Granada, 18071 Granada, Spain

article info

abstract

Article history:

Biosorption of Pb(II) by olive tree pruning (OTP) treated with three chemical agents (HNO3,

Received 2 January 2013

H2SO4 and NaOH) using a batch system was studied. The effect of pH over a range from 3.0

Received in revised form

to 5.0, the initial lead concentration over a range from 50 to 250 g m
3 and the treatment

15 July 2013

concentration over a range from 100 to 2000 mol m
3 on the metal removal was investi-

Accepted 13 August 2013

gated. All treatments improved the percentage of lead removal by native OTP. Improve-

Available online 12 September 2013

ments obtained with treatments of treated OTP respect to untreated OTP at pH 5, an initial lead concentration of 250 g m
3 and treatment at 1000 mol m
3 were 64.72, 55.05 and

Keywords:

64.82% to HNO3, H2SO4 and NaOH respectively. The metal uptake also increased by

Valorizing of waste

increasing the pH and the initial lead concentration. A 33 factorial experimental design was

Biosorption

employed to study the effect of three factors (pH, initial lead concentration and treatment

Chemical treatment

concentration) for olive tree pruning treated with three treatment (HNO3, H2SO4 and NaOH)

Lead

and at three levels (low, intermediate and high). A second-order polynomial regression

Olive tree pruning

model was used to fit experimental data obtained, and results showed a good agreement with experimental data for the three cases studied. Results obtained for the three treatments were compared and the best response was obtained with OTP treated with NaOH. Optimization process predicted the optimum conditions for the maximum biosorption capacity of Pb(II) for each treatment: 25.54, 23.87 and 26.63 g kg
1 for treatments with HNO3, H2SO4 and NaOH respectively. The equilibrium data for biosorption of lead onto olive tree pruning were represented by the Langmuir and Freundlich isotherms, giving the best fit the Langmuir model. ª 2013 Elsevier Ltd. All rights reserved.

1.

Introduction

During recent years, the surge of industrial activities has led to a tremendous increase in the use of heavy metals and

inevitably resulted in an increased flux of these metal ions in the aquatic environment. Cu, Cr, Cd, Pb, Hg, Zn and Ni are the most common heavy metals discharged into water streams from large industrial sectors [1e4]. These substances cannot be degraded and are accumulated in the environment (soils,

* Corresponding author. Tel.: þ34 958 243311; fax: þ34 958 248992. E-mail addresses: [email protected] (M. Calero), [email protected] (A. Ronda), [email protected] (M.A. Martı´n-Lara), [email protected] (A. Pe´rez), [email protected] (G. Bla´zquez). 1 Tel.: þ34 958 243311; fax: þ34 958 248992. 2 Tel.: þ34 958 240445; fax: þ34 958 248992. 3 Tel.: þ34 958 240770; fax: þ34 958 248992. 0961-9534/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2013.08.021

b i o m a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 2 2 e3 3 2

water and sediments). Lead is one of the most toxic heavy metals, existing in several industrial and mining wastes, such as lead acid storage battery, solders, painting, pigments, pesticides, lead smelting slag and mine tailings [5]. Lead is a highly toxic and cumulative poison, can damage the nervous system, kidneys, and reproductive system, particularly in children. The EPA requires lead in drinking water not to exceed 0.015 g m
3. Therefore, all industrial effluents, containing lead ions should be disposed to reduce Pb(II) to acceptable level. By the moment, various treatment technologies have been used for removal of Pb(II) from wastewater, such as precipitation, ion-exchange, evaporation, oxidation, electroplating and membrane filtration [6e8]. Nevertheless, these conventional methods have several disadvantages, such as high installation and operating costs, difficulty of treating the subsequently generated solid waste, low efficiency at low metal concentration (<100 g m
3) and unpredictable metal ion removal [9]. Biosorption as an alternative and effective technology has been widely studied over recent years, because of its wide range of target pollutants, high sorption capacity, excellent performance, ecofriendly nature and low operating cost [10e13]. In recent years, different types of agricultural waste have been used for the removal lead such as: olive cake [14], olive stone waste [15,16], almond shell [17,18], grape stalk waste [19], shells of hazelnut [18], modified wheat straw [5], cork waste [20], agricultural bean husk powder [21], etc. The olive tree pruning is an agricultural by-product in the Mediterranean region. Until now, there were not found technological and economic applications viable for the olive tree pruning. Although there are some studies about its use for thermochemical and biochemical conversion, its most frequent use is disposal on land crop by burning. However, this practice has a lot of disadvantages (such as propagation pest, mineralization soil, air pollution, .). So, researches about new uses for waste of olive oil industry are very important in the communities where olives grow. This paper proposes other different use of olive tree pruning, as biosorbent to removal Pb(II) from aqueous solutions. Although olive tree pruning was previously investigated as biosorbent [22e24] and these research activities indicated promising results, further efforts are still required in order to maximize metal removal efficiency and reduce problems of the application of untreated olive tree pruning. In this study, the olive tree pruning was chemically modified with three agents (HNO3, H2SO4 and NaOH) at three different treatment concentrations (100, 1000 and 2000 mol m
3). The biosorption was evaluated at three pH (3, 4 and 5) and at three initial lead concentrations (50, 150 and 250 g m
3). Experiments were performed using the batch mode biosorption. The optimal condition for each treatment was chosen by an experimental design analysis. The aim of this work was to evaluate the significant effects of different treatment procedures on lead removal using olive tree pruning. A factorial design and an optimization analysis were carried out in order to observe improvements on biosorption capacity of OTP after pre-treatment. The predicted result by the fitting model was then compared with the experimental results. Finally, the equilibrium of the biosorption

323

of lead onto treated olive tree pruning was studied by using Langmuir and Freundlich models.

2.

Materials and methods

2.1.

Biomass

Olive tree pruning (OTP) was obtained of the olive tree pruning process in Jaen (Spain). The olive cultivar was located in Vilchez (38 120 000 N, 3 300 000 W), with a total area of 25000 m2. The specie of olive is “picual” and with 70 years old. The pruning process was performed in FebruaryeMarch. In the laboratory, biomass was milled with an analytical mill (IKA MF-10) and <10
3 m fraction was chosen for the lead biosorption tests.

2.1.1.

Modification of biomass

The chemical modification of OTP was performed by using three chemical solutions: nitric acid (HNO3), sulfuric acid (H2SO4) and sodium hydroxide (NaOH). The solutions for treatment were prepared at different concentration (100, 1000 and 2000 mol m
3) to analyze the effect of treatment concentration in the biosorption capacity. 10
3 m3 of these solutions were used to treat 10 g of biomass in a flask at constant temperature (50  C). Biomass and chemical solution were mixed for 86,400 s to establish complete contact. After that, the biomass was repeatedly washed with distilled water until the pH of rising water remained constant. The treated OTP was then dried in an oven at 40  C for 86,400 s and after stored for later use.

2.2.

Preparation of lead solutions

A stock solution of 2 kg m
3 Pb(II) was prepared by a dissolving desired amount of Pb(NO3)2in 500 cm
3 of distilled water. After that, solutions of different concentrations were prepared by appropriate dilution of the above stock Pb(II) solution.

2.3. Characterization of untreated and treated olive tree pruning Untreated and treated OTP were subjected to different analysis to their characterization in previous studied. A review of results obtained are shown in Table 1.

3.

Theory

3.1.

Batch experiments

The adsorption experiments were conducted in a batch system with a 150 cm
3 flask in a thermostatic shaker (25  C). The experiments were performed by mixing 1 g of biomass in 100 cm
3 of metal solutions at different concentrations of lead. The pH was initially adjusted to the desired value and it was kept constant with 100 mol m
3 HCl and 100 mol m
3 NaOH solutions. After 7200 s the final lead concentrations were measured using absorption spectrophotometer (PerkineElmer, model 3100).

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Table 1 e Characteristics of biosorbents. Treatment of OTP Physical characterization BET surface area, m2 kg
1 Pore volume, m3 kg
1 Pore diameter, nm Particle size, mm Composition of biosorbents Hot water soluble compounds, g kg
1 Ethanol-benzene extractives compounds, g kg
1 Lignin, g kg
1 Holocellulose, g kg
1 Proximate analysis Moisture, g kg
1 Volatile material, g kg
1 Fixed carbo´n, g kg
1 Ash, g kg
1 Potentiometric titrations Total titratable sites, mol kg
1 Acid titratable sites, mol kg
1 Basic titratable sites, mol kg
1 Point of zero charge (pHPZC) Loss of biomass Loss of biomass, %

Main groups FTIR analysis OeH

Wavenumbers, A, % Wavenumbers, A, % Wavenumbers, A, % Wavenumbers, A, % Wavenumbers, A, %

CeH C¼O CeO CeO alcoholic

cm
1 cm
1 cm
1 cm
1 cm
1

UntreatedOTP

HNO3OTP

H2SO4OTP

NaOHOTP

631.2 1.5  10
6 9.744 <1

425.6 1.0  10
6 9.188 <1

611.2 1.6  10
6 10.80 <1

3526.2 5.9  10
6 6.638 <1

392.6

127.0

211.6

124.8

35.2

79.5

97.3

25.3

170.8 401.4

217.2 576.3

240.8 450.3

149.3 700.6

53.6 769.3 120.4 56.7

e 813.8 166.7 19.5

e 842.0 152.3 5.7

e 764.2 160.4 75.4

0.525 0.278 0.247 5.24

1.36 1.34 1.87 3.23

0.648 0.648 0 3.54

0.491 0 0.525 6.77

e

35.1

27.5

46.7

UntreatedOTP

HNO3OTP

H2SO4OTP

NaOHOTP

3318.5 3.34 2918.8 3.79 1729.6 3.06 1239.8 4.93 1027.7 10.17

3331.8 3.78 2898.6 2.32 1737.1 1.87 1228.2 4.27 1027.5 11.53

3338.9 3.74 2894.3 2.30 1729.3 2.02 1227.2 4.70 1027.9 12.20

3336.8 9.51 2915.5 3.77 e e 1225.1 6.58 1028.5 18.74

The amount of Pb(II) biosorption at equilibrium qe (mg g
1) was calculated according to the following mass balance equation for the metal ion concentration: qe ¼

ðCi
Ce Þ$V m

(1)

where Ci is the initial Pb(II) concentration (mg L
1), Ce is the equilibrium Pb(II) concentration in solution (mg L
1), V is the volume of the solution (L), and m is the mass of the biosorbent used (g). The percent sorption (%) of Pb(II) was calculated using the equation (2): Sorptionð%Þ ¼

3.2.

Ci
Ce $100 Ci

(2)

Modeling by full factorial design

To develop a model for lead biosorption, some operating parameters such as pH, initial lead concentration and chemical treatment concentration were studied. Factorial designs allow

the simultaneous study of the effects that several factors may have on the optimization of a particular process with less number of experiments [25,26]. It determines which factors have the important effects on the response as well as how the effect of one factor varies with the level of the other factors. The effects are the differential quantities expressing how a response changes as the levels of one or more factors are changed. Also, factorial designs allow measuring the interaction between each different group of factors. The factorial design analysis was performed for data obtained with HNO3, H2SO4 and NaOH treatments, and the response chosen to the study was the biosorption capacity of Pb(II) by treated OTP (Y). In this paper, a factorial design of three levels (low:
1, intermediate: 0 and high: þ1) and three factors (X1: pH, X2: initial concentration of Pb(II) and X3: concentration of solution of treatment) was used in experimental work. Other variables such as the contact time or the solution volume were fixed at 7200 s and 10
4 m3, respectively (same that in all others batch experiments). Then, 27 (33) measurements are required to perform a factorial design analysis.

b i o m a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 2 2 e3 3 2

Table 2 shows values and levels of operating parameters. The factorial design analysis was performed for olive tree pruning with three treatments, and then, results were compared.

3.3.

Biosorption equilibrium

Data obtained at optimal conditions were subject to equilibrium modeling to have a better understanding of Pb(II) biosorption mechanism using two models isotherms such as Langmuir and Freundlich. The Langmuir isotherms assumes that biosorption takes place at specific homogeneous sites on the surface of the adsorbent, meaning once a lead molecule occupies a binding site, no further biosorption can occur at that site (Langmuir, 1918) [27]. The linear form of the Langmuir isotherm model can be presented as: Ce 1 1 þ ¼ $Ce qe qmax $b qmax

(3)

where Ce is the equilibrium lead concentration in the solution (mg L
1), qe is the equilibrium lead uptake on the biosorbent (mg g
1), qmax is the maximum biosorption capacity (mg g
1), and b is the Langmuir constant that is related to the affinity of binding sites and is related to the energy of sorption, (L mg
1). The Freundlich isotherm model is valid for multilayer biosorption on a heterogeneous adsorbent surface with a no uniform distribution of heat of adsorption (Freundlich, 1906) [28]. The equation of Freundlich isotherm is given as below: 1 log qe ¼ log KF þ $log Ce n

(4)

where KF (L g
1) and n (dimensionless) are characteristic constants that indicate the extent of the biosorption, the degree of non-linearity between solution concentration and biosorption, respectively.

4.

Results and discussions

4.1.

Effect of pH on metal uptake

The pH of the solution has a significant effect on the heavy metal uptake since it controls the extent of surface protonation of the sorbent and the degree of ionization [29]. The diagram of lead species in solution (Figure not shown) was obtained before to analyzing the effect of pH onto

Table 2 e Values and levels of operating parameters. Factors

X1: pH X2: Initial concentration of lead (kg m
3) X3: Concentration of treatment solution (mol m
3)

Levels
1

0

3 0.050

4 0.150

100

1000

þ1 5 0.250

2000

325

biosorption capacity. It shows that the lead precipitated as Pb(OH)2 at pH higher than 5.5 being the process of retention really a combination of biosorption and microprecipitation. The diagram shows that when the pH is lower than 5.5 the main specie in the solution is Pb(II). For this reason the study of pH was performed for pH lower than 5.5. Metal uptake by the chemical treated OTP at three concentrations was studied from pH 3.0 to 5.0 with three metal concentrations during a contact time of 7200 s was studied. As an example, in Fig. 1 are shown results obtained with an initial lead concentration of 150 g m
3. It is observed that very high percentages of lead removal were obtained for three studied pH, however the % of metal removal increases slightly when the pH increases. This effect is common to all treatments performed to OTP. The optimal pH chosen for the following experiments was 5. At this pH, all pretreatments shown a very high biosorption capacity (practically the 100% of lead is retained) and it ensures that there is no microprecipitation process.

4.2.

Effects of chemical treatment

Experiments with OTP treated with three chemical treatments (HNO3, H2SO4 and NaOH) at three treatment concentrations and three initial lead concentrations (50, 150 and 250 g m
3) were performed to study the effect of chemical treatment in the biosorption process. Data obtained from treatment studies were compared with data obtained from untreated OTP [30]. As an example, to analyze better the influence of chemical treatments on sorption capacity of OTP in Table 3 are shown results obtained with a treatment concentration of 1000 mol m
3. All treatments improved the percentage of lead removal by OTP. When the initial lead concentration was 250 g m
3, the obtained improvements on biosorption of lead with treated OTP compared to untreated OTP were of 54.85, 239.90 and 255.00% for treatments with HNO3, H2SO4 and NaOH respectively. The improvements obtained are also observed in Fig. 2. It shows the percentage of lead removal by treated OTP with three chemical agents at three treatment concentrations and three initial lead concentrations. The black line indicates the percentage of lead removal by untreated OTP. It is observed that for all conditions, treatments improve the biosorption capacity of untreated OTP. Improvements observed by OTP when the biosorbent was treated with chemical agents were similar to results found by other authors. A summary of some of this results obtained by them is shown in Table 4. Most of them agree that the chemical treatment of biomass changes the biosorption capacity of biosorbent, modifying the physicochemical characteristics of the biosorbent. For this reason, the physical and chemical characterization of biosorbent is vital to understanding the metal binding mechanism onto biomass. Table 1 shows the characterization of OTP and the main changes produced with treatments. Respect to physical properties, treatment with NaOH multiplied by five the specific area of raw OTP and triples its total pore volume. In contrast, treatment with nitric acid decreases the surface area and total pore volume and on the other hand, sulfuric acid treatment practically does not modify the physical properties of native

326

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100

100

OTP-H SO

OTP-HNO

80

C = 150 g·m

C = 150 g·m

Removal, %

Removal, %

80

60

40

60

40

20

20

0

0

2.5

2.5

3.0 100 mol·m 1000 mol·m 2000 mol·m

3.5

4.0

4.5

5.0

3.0

3.5

5.5 100 mol·m 1000 mol·m 2000 mol·m

pH

4.0

4.5

5.0

5.5

pH

100

OTP-NaOH C = 150 g·m

Removal, %

80

60

40

20

0 2.5

3.0

3.5

100 mol·m 1000 mol·m 2000 mol·m

4.0

4.5

5.0

5.5

pH

Fig. 1 e Effect of pH. a) OTP treated by HNO3; b) OTP treated with H2SO4; c) OTP treated with NaOH.

biosorbent. It may be due to the NaOH treatment attack more to the biosorbent, disintegrating the particles. Elemental analysis results shown that the composition of untreated and treated OTP were similar. Treatments do not affect to percentages of C, N, O and H, only change the structure of biosorbent. Results of the proximate analysis noted that the ash content decreased when the OTP was treated with acid treatments, while with the basic treatment this content increased. This can be due to the reactive of basic treatment has a metal (Na), and it can remain as residue in the structure of OTP treated by NaOH. About chemical composition, experiments involved that hot water soluble compounds decreased with treatments, due to during the treatment some soluble components were dissolved. With the acid treatment the ethanol-benzene extractives compounds increased, it can be due to that treatment modified compounds initially present in the sample, making them soluble in ethanol-benzene. However, the basic treatment did not modify compounds, and the percentage of them decreased.

About lignin contents, it noted that acid treatments increased relative amount of lignin, because during treatments, cellulose was dissolved, not affecting to lignin. However, basic treatment attacks to lignin, removing these compounds. Finally, comparing holocellulose content, it is observed that treatments with HNO3 and NaOH increased considerably these compounds, indicating that these treatments do not practically affect to the holocellulose, and the mass loss was due to the loss of the other types of compounds. It can be due to each treatment affect mainly to a type of compounds (lignin or cellulose), and depending of treatment, the relative amount of this compounds increase or decrease. On the other hand, the data for the acid-treated OTP samples illustrate that chemical treatment of OTP by acids developed negatively charged groups (such as carboxylic groups), especially when OTP is treated by HNO3. Therefore, their surfaces exhibited a slightly acidic character (acid treatment clearly lowered the pHPZC). However, although treatment of OTP by NaOH gave a biosorbent with a

327

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Table 3 e Percentages of lead removal by untreated OTP and treated OTP with three treatments at 1000 mol mL3 and at three different initial lead concentrations. [Pb]i, kg m
3 0.050 0.150 0.250

Untreated OTP

HNO3OTP

H2SO4OTP

NaOHOTP

90.53 75.79 60.14

100 94.90 99.06

100 92.79 93.25

97.53 100 99.12

4.3.

Effects of the concentration of chemical treatment

OTP was treated with three different concentrations of chemical agents (100, 1000 and 2000 mol m
3). Results are shown in Figs. 1 and 2. It is observed that obtained results for each treatment are different and a clear trend is not observed. So, in general it can be said that results obtained with concentrations of 100 and 1000 mol m
3provided similar results, while those

100

100

80

80

Removal, %

Removal, %

total number of titratable sites similar to that of untreated OTP, when OTP was treated with NaOH, the OTP assumed a basic character with a pHPZC value of 6.77 due to a significant increase of basic titratable sites and a total decrease of acid

titratable sites. Finally, in the FTIR analyzes shown the presence of carboxyl and hydroxyl groups characteristic of these types of waste [13], in both, untreated and treated OTP. Highlighting the disappearance of the carbonyl group in the basic treatment.

60

40

20

60

40

20

0

0 0

50 HNO 100 mol·m

100

150

200

250

300

0

Ci Pb, g·m-3

50

100

H SO 100 mol·m

HNO 1000 mol·m

H SO 1000 mol·m

HNO 2000 mol·m

H SO 2000 mol·m

Untreated

Untreated

150

200

250

300

-3

Ci Pb, g·m

100

Removal, %

80

60

40

20

0 0

50

100

NaOH 100 mol·m-3 -3 NaOH 1000 mol·m NaOH 2000 mol·m-3 Untreated

150

200

250

300

Ci Pb, g·m-3

Fig. 2 e Effect of initial lead concentration and concentration of chemical treatment. a) OTP treated by HNO3; b) OTP treated with H2SO4; c) OTP treated with NaOH.

328

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Table 4 e A summary of some results obtained by other authors with treated biosorbents. Adsorbent

Compound removed

Citrus sinensis

Rhizopus arrhizus Mucor hiemalis Pine cone

Reactive Reactive Reactive Reactive Cr(VI) Ni(II) Cu(II)

Olive stone

Pb(II)

Alfalfa biomass Olive tree prunning

Pb(II) Pb(II)

Treatment

yellow 42 blue 19 blue 49 red 45

qe (mg/g)

qe of untreated adsorbent (mg/g)

17.64 23.31 33.53 18.18 31.52 13.60 14.85 15.73 17.22 7.84 14.03 10.01 3.89 89.2 84.034 50.4761 121.951

13.99 15.21 14.80 27.41 21.72 5.67 6.80

[32] [9] [33]

1.31

[16]

Acetic acid

Nitric acid Sodium carbonate Calcium hydroxide Potassium hydroxide Sodium hydroxide Chloric acid Sulfuric acid Nitric acid Phosphoric acid Sodium hydroxide Sulfuric acid Nitric acid Sodium hydroxide

one obtained with the concentration of 2000 mol m
3gave worse results. In all cases (for the three treatment concentrations) the percentage of lead removal increased compared to results obtained with untreated OTP.

4.4.

Reference [31]

43.0 22.8

[34] This study

Application of factorial design analysis

The natural parameters and coded values of variables with the value of studied response (biosorption capacity) for OTP

Table 5 e Full factorial design matrix for treated OTP. Natural and coded values of parameters. Runs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Natural values of parameters

Y (mg g
1)

Coded values of parameters

pH

Initial lead concentration (kg m
3)

Concentration of lead solution (mol m
3)

X1

X2

X3

HNO3-OTP

H2SO4-OTP

NaOH-OTP

3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5

0.050 0.150 0.250 0.050 0.150 0.250 0.050 0.150 0.250 0.050 0.150 0.250 0.050 0.150 0.250 0.050 0.150 0.250 0.050 0.150 0.250 0.050 0.150 0.250 0.050 0.150 0.250

100 100 100 1000 1000 1000 2000 2000 2000 100 100 100 1000 1000 1000 2000 2000 2000 100 100 100 1000 1000 1000 2000 2000 2000


1
1
1
1
1
1
1
1
1 0 0 0 0 0 0 0 0 0 þ1 þ1 þ1 þ1 þ1 þ1 þ1 þ1 þ1


1 0 þ1
1 0 þ1
1 0 þ1
1 0 þ1
1 0 þ1
1 0 þ1
1 0 þ1
1 0 þ1
1 0 þ1


1
1
1 0 0 0 þ1 þ1 þ1
1
1
1 0 0 0 þ1 þ1 þ1
1
1
1 0 0 0 þ1 þ1 þ1

5.09 14.59 15.77 4.28 13.88 19.05 5.98 14.79 23.43 4.44 15.01 24.92 4.81 14.67 24.90 4.76 13.00 24.37 4.46 18.64 24.66 4.34 14.33 23.18 4.64 14.22 25.62

5.40 14.00 17.52 5.38 13.45 22.48 5.38 15.47 25.27 5.44 16.87 26.44 5.04 14.19 25.60 5.31 15.82 26.68 4.99 15.53 24.72 6.13 15.18 25.27 5.45 14.75 20.57

5.04 16.59 25.22 5.75 15.02 26.45 4.61 14.54 24.80 5.34 15.51 26.55 4.86 16.20 27.30 4.61 14.50 25.78 5.07 16.30 24.67 5.13 15.66 26.98 3.19 12.79 20.79

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Table 6 e Estimated model and their statistical significance for biosorption capacity of Pb(II) by OTP with three treatments. Treatment

Equation model Y ¼ 14.766 þ 0.957∙X1þ9.061∙X2þ 0.179∙X3
0.601∙X21 þ1.427∙X1∙X2
1.003∙X1∙X3
0.976∙X22 þ 0.557∙X2∙X3þ0.639∙X23 S ¼ 1.472, R2 ¼ 97.08%, R2(adj.) ¼ 96.48% Y ¼ 15.935 þ 0.424∙X1 þ 9.191∙X2 þ 0.244∙X3
1.402∙X21 þ0.457∙X1∙X2
1.189∙X1∙X3
0.381∙X22 þ 0.244∙X2∙X3þ0.043∙X23 S ¼ 1.509, R2 ¼ 96.95%, R2(adj.) ¼ 96.33% Y ¼ 16.490
0.408∙X1 þ 10.274∙X2
0.816∙X3
0.700∙X21
0.168∙X1∙X2
0.531∙X1∙X3
0.127∙X22
0.169∙X2∙X3þ1.167∙X23 S ¼ 0.865, R2 ¼ 99.17%, R2(adj.) ¼ 99.00%

HNO3

H2SO4

NaOH

treated with three treatments (HNO3, H2SO4 and NaOH) are illustrated in Table 5. A factorial design analysis was performed, and the factors which have a more important effect onto the responses were studied by Pareto and Main effect plots (Supplementary material). In addition, a regression analysis were performed to fit the response function to the experimental data. Obtained values of constants were 14.766, 15.935 and 16.490 for treatment with HNO3, H2SO4 and NaOH respectively. A second-order regression equation was prepared for each treatment using the coefficients obtained in the fit (which are given in Table 6). These equations have the following form: Y ¼ a þ b$X1 þ c$X2 þ d$X3 þ e$X21 þ f $X1 $X2 þ g$X1 $X3 þ h$X22 þ i$X2 $X3 þ j$X23

4.5.

140 HNO 100 mol·m

qe, mg·g

Table 7 e Optimal values predicted by the model for biosorption capacity of OTP with three treatments, and codified values of the factors studied. HNO3-OTP

H2SO4-OTP

NaOH-OTP

25.54

23.87

26.63

Biosorption isotherms

Data obtained in isotherm experiments were analyzed with Langmuir and Freundlich models to explicate the biosorption mechanism. Analyses were performed with data obtained from biosorption of Pb(II) by OTP treated with the three chemical agents (HNO3, H2SO4 and NaOH) at optimal condition for each one. That is, OTP treated with HNO3 at 100 mol m
3 and pH 5; OTP treated with H2SO4 at 2000 mol m
3 and pH 4 and OTP treated with NaOH at 1000 mol m
3 and pH 4. The experimental isotherms data for these treatments of OTP at 25  C are graphically shown in Fig. 3. Results shown that the best treatment, according to the biosorption capacity, was the treatment of OTP with NaOH, obtaining values for maximum biosorption capacity about 120 mg g
1 (at optimal conditions). Acid treatments provided lower results, values of biosorption capacity around of 45 and 80 mg g
1 for HNO3 and H2SO4 treatment respectively, although they also improved results of untreated OTP.

-1

where Y is the response variable, the predicted biosorption capacity of treated OTP (mg g
1), X1, X2 and X3 parameters studied and a, b, c, d, e, f, g, h, i and j values of constants obtained. Values of standard deviation, R2 and R2 (adjusted) are also given in Table 6. It is observed high values of R2 (97.08, 96.95 and 99.17%) and low values of standard deviation (1.472, 1.509 and 0.865) for HNO3, H2SO4 and NaOH treatments respectively. They indicated a high dependence and correlation between the observed and the predicted values of biosorption capacity of Pb(II) by OTP. Values of standard deviation and of R2 indicate that the model with better results are for OTP treated with

NaOH, because of, it presents the lowest value for SD and the highest one for R2. An analysis of variance (ANOVA) was carried out, and the statistical significance of the ratio of mean square due to regression and mean square due to residual error was studied. Results obtained are shown in Supplementary material. Finally, the maximum biosorption capacities for treated OTP were predicted with the help of response surfaces and predicted models. Optimal values of biosorption capacity obtained were 25.54, 23.87 and 26.63 mg g
1, for treatments with HNO3, H2SO4 and NaOH respectively. And the optimal conditions obtained for each treatment were: for HNO3treatment, pH 5, initial lead concentration of 250 g m
3 and treatment concentration of 100 mol m
3; for H2SO4 treatment, pH 4, initial lead concentration of 250 g m
3 and treatment concentration of 2000 mol m
3 and for NaOH treatment, pH 4, initial lead concentration of 250 g m
3 and treatment concentration of 2000 mol m
3. Codified values for three factors studied in optimum conditions are shown in Table 7.

120

H SO 2000 mol·m

100

NaOH 1000 mol·m Untreated

80

60

40

20

0

Optimal qe, mg g
1

500

1000

1500

2000

2500

-3

Ce, g·m

Factors X1 X2 X3

0

Codified values þ1 þ1
1

0 þ1 þ1

0 þ1 0

Fig. 3 e Experimental isotherms for Pb(II) removal (pH 5, OTP dose of 10 g LL1 and contact time of 120 min) at optimum conditions for each treatment. OTP treated by HNO3; OTP treated with H2SO4; OTP treated with NaOH.

330

b i o m a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 2 2 e3 3 2

Table 8 e The Langmuir and Freundlich parameters for OPT treated at optimal conditions. Langmuir

Untreated HNO3 H2SO4 NaOH

100 mol m
3 2000 mol m
3 1000 mol m
3

2

Freundlich P

r2

23.35 1411.09

3.508 27.060

3.31 12.21

0.986 0.852

37.55 1341.59

0.992

479.01

2.923

2.03

0.923

77.17

0.991

1824.53

15.367

3.02

0.912

2669.69

r

22.8 50.761

0.0347 0.062

0.993 0.996

84.034

0.011

121.951

0.044

ðqexp
qcal Þ

for OTP, not only the increase in the biosorption capacity of treated OTP respect untreated OTP, but also, the mass lost during the treatment, specially for an industrial application, must be taken into account. Both parameters indicate the viability of these treatments. With the objective to perform a more exhaustive analysis more exhaustive of data, values of maximum biosorption capacity were obtained taking into account these lost. So, in Table 9 a relative value of biosorption capacity (referred to 1 g of natural biosorbent, before of treatment), qer, and the relation between these values and biosorption capacity of untreated OTP, qes are shown. It is observed that in all cases, the relation between relative biosorption capacity (qer) and untreated biosorption capacity (qes) were higher than one, reaching a value of 2.47 for OTP treated with NaOH. These results confirm, firstly, better results for OTP were obtained with all treatments, and secondly, as indicated above, that the best treatment of OTP was with NaOH.

5.

Finally, the loss of biomass with the treatment was analyzed. In Table 1 are given the biomass loss for each treatment, with values between 30 and 50%. It is observed that the loss is important, reaching values nearly of 50% for treatment with NaOH. These results are similar to results found by other authors in the treatment of solids with different chemical agents: Shorff and Vaidya treated dead biomass of Rhizopus arrhizus with different chemical solutions, and for all them, loss in treatment were about 20e40% [32]; when the treatment of dead biomass of Mucor hiemalis was with basic solutions loss of biomass were in a higher range, between 20 and 80% [9]. Thus, according to the evaluation of optimal treatment

Table 9 e Values of maximum biosorption capacity obtained taking into account the loss of biomass.

Untreated HNO3 100 mol m
3 H2SO4 2000 mol m
3 NaOH 1000 mol m
3

ðqexp
qcal Þ2

n

b

Loss of biomass

Treatment

P

KF

qm

Data obtained in isotherm experiments were analyzed with Langmuir and Freundlich models to explicate the biosorption mechanism. A linear regression and a statistical analysis were used to determine the best-fitting isotherm. The Langmuir, Freundlich parameters are presented in Table 8, demonstrating that the Langmuir equation provides a reasonable description of the experimental data (values or R2 were higher than 0.99 in all cases). Besides, the Langmuir isotherm exhibits higher R2 values and lower SSE values, showing it to be a considerably better fit compared with the Freundlich isotherm for all chemical treatment of olive tree pruning. Comparing the results of biosorption capacities of OTP treated with the obtained for untreated OTP, 26.7 mg g
1 [30], chemical treatments of biomass produce a significant increase of biosorption capacity in all the cases. The monolayer capacities of OTP were 22.8 mg g
1, 50.76 mg g
1, 84.03 mg g
1 and121.95 mg g
1, for untreated OTP and treated OTP with HNO3, H2SO4, NaOH at optimal conditions for each one. The Langmuir constant, b, was 0.0347, 0.062, 0.011 and 0.044 respectively.

4.6.

2

qe, mg g
1

qer, mg g
1

qer/qes

26.70 51.13 79.80 123.80

26.70 33.39 50.67 65.98

1.00 1.25 1.90 2.47

Conclusions

Results of this study shown that all chemical treatments on OTP improved its capacity of biosorption of Pb(II) and the percentage of lead removal, with values of percentage of lead removal higher than 80%. At initial lead concentration of 250 g m
3, treatments improved more than 50% the amount of lead removal respect to untreated OTP. The study of different parameters showed that biosorption capacity of OTP depended of the solution pH, initial lead concentration and treatment concentration, being the most significant factor the initial lead concentration. The biosorption capacity increased with increasing of pH and initial lead concentration, while the treatment concentration had not a clear trend. A 33 factorial design had been found between the experimental and predicted results for the response studied in all cases, exposing a good correlation (R2 values of models higher than 95%). Models were adequate to represent the response surface and to obtain the optimal conditions for biosorption capacity of treated OTP, being the optimal conditions obtained for each treatment were: pH 5, initial lead concentration of 250 g m
3 and treatment concentration of 100 mol m
3 for OTP treated with HNO3; pH 4, initial lead concentration of 250 g m
3 and treatment concentration of 2000 mol m
3 for OTP treated with HNO3; and pH 4, initial lead concentration 250 g m
3 and treatment concentration of 1000 mol m
3 for OTP treated with

b i o m a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 2 2 e3 3 2

NaOH. The study of biosorption isotherms shown that two models studied (Langmuir and Freundlich model) represented appropriately the experimental results. The Langmuir model provided an excellent fit of the experimental data (R2 > 0.99) in all studies case. Finally, taking into account the loss of biomass, it can be seen that the biosorption capacity also improved in all cases, being treatment with NaOH which provided the best results.

Acknowledgments The authors are grateful to the Spanish Ministry of Science and Innovation for financial support received (Project CTM2009-10294).

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.biombioe.2013.08.021.

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